overview of versatile experiment spherical torus …...trapped particle configuration (tpc)...

Experimental results and plans of VEST

Y.S. Hwang and VEST team

March 29, 2017

CARFRE and CATS

Dept. of Nuclear Engineering Seoul National University

Overview of Versatile Experiment Spherical Torus (VEST)

23rd IAEA Technical Meeting on Research Using Small Fusion Devices, Santiago, Chille

1/36

Versatile Experiment Spherical Torus (VEST)

Device and discharge status

Start-up experiments

Low loop voltage start-up using trapped particle configuration

EC/EBW heating for pre-ionization

DC helicity injection

Studies for Advanced Tokamak

Research directions for high-beta and high-bootstrap STs

Preparation of long-pulse ohmic discharges

Preparation for heating and current drive systems

Preparation of profile diagnostics

Long-term Research Plans

Summary

Outline

2/36

VEST device and Machine status

VEST device and

Machine status

3/36

Present Future

Toroidal B Field [T] 0.1 0.3

Major Radius [m] 0.45 0.4

Minor Radius [m] 0.33 0.3

Aspect Ratio >1.36 >1.33

Plasma Current [kA] ~100 kA 300

Elongation ~1.6 ~2

Safety factor, qa ~6 ~5

Specifications

VEST (Versatile Experiment Spherical Torus)

Objectives

Basic research on a compact, high- ST (Spherical Torus)

with elongated chamber in partial solenoid configuration

Study on innovative start-up, non-inductive H&CD, high and

innovative divertor concept, etc

4/36

History of VEST Discharges • #2946: First plasma (Jan. 2013) • #10508: Hydrogen glow discharge cleaning (Nov. 2014) Ip of ~70 kA with duration of ~10 ms • #14945: Boronization with He GDC (Mar. 2016) Maximum Ip of ~100 kA

• # ??: Long pulse with coil power supply upgrade (March. 2017)

5/36

VEST Discharge Status : 100kA with Boronization (#14945) Discharge Status (Shot #14945)

• EC-assisted OH discharge

– Plasma current 100 kA

– Hydrogen plasma with Boronization

– Tungsten & Graphite (inboard) limiter

• Equilibrium parameters

– R=0.45m/a=0.33m (A=1.36)

– Elongation: 1.6 – 2.0

– Edge safety factor: ~ 7

• Limited discharge length

– Fast ramp-up Require large Bv

in the beginning

– Wall eddy current Limited time

response of Bv

– Excessive vertical field near current peak early termination

6/36

Start-up Experiments

Start-up Experiments

7/36


Widely used Field Null Configuration (Breakdown/avalanche)

The field null configuration has been generally used even for the ECH-assisted start-up of tokamaks.

KSTAR

[Y.S. Bae et al 2009 Nucl. Fusion 49 022001]

Frascati Tokamak Upgrade

[G. Granucci et al, Nucl. Fusion 51 (2011) 073042]

Field null formation to maximize connection length and consequently sufficient EtBt/Bp

Null quality : size, sustaining time and location loop voltage and pre-ionization

Notes:

KSTAR: not much help with higher ECH power in the ECH-assisted start-up

DIII-D: vertical field pre-bias helps ohmic start-up with ECH pre-ionization

DIII-D

[NUCLEAR FUSION, Vol. 31. No. 11 (1991)]

8/36


Trapped Particle Configuration (TPC)

Efficient and robust tokamak start-up demonstrated

Y. An et al., Nucl. Fusion 57 016001 (2017)

Trapped particle Field null

• Mirror like magnetic field configuration – Enhanced particle confinement – Inherently stable decay index structure – Wider operation range of ECH power, pressure and low Vloop

Start-up Experiment in VEST

Robust and Reliable TPC Start-up Applied to KSTAR Successfully

Feasibility study of TPC in KSTAR

• Even though low mirror ratio than ST, achieving efficient start-up with TPC

• 2nd harmonic delay of 20 ms and ECH plasma density of 4x1018 m-2

• Ip formation with low Et less than 0.2 V/m

Earlier plasma colomn formation

than field null

J. Lee, et al., Proc. IAEA-FEC 2016 EX/P4-14

10/36


Low Loop Voltage Start-up with different ECH Power at Low TF Field in TPC

Earlier Ip initiation at low loop

voltage with larger ECH power in

double swing TPC start-up

Smaller resistivity initiate higher

initial current to form sufficient

size of closed flux surface

Threshold current for successful

inductive start-up (reference for

outer PF start-up)

H.Y. Lee

11/36

EC / EBW pre-ionization

VEST pre-ionization with LFS XB mode conversion

0 0.2 0.4 0.6 0.8 1-1.5

-1

-0.5

0

0.5

1

1.5Coil Geometry

R (m)

Z (

m)

2.45GHz

6kW CW

ECH

J.G. Jo 30 35 40 45 50 55 60 65 70 75 80 85

3

6

9

12

15

18

21

24

Te [

eV

]

R [cm]

X-mode_2kW

X-mode_3kW

X-mode_4kW

X-mode_6kW

ECR

30 35 40 45 50 55 60 65 70 75 80 850.0

0.2

0.4

0.6

0.8

1.0

1.2

nUH

nR

ne [10

17m

-3]

R [cm]

X-mode_2kW

X-mode_3kW

X-mode_4kW

X-mode_6kW

ECR

12/36

Start-up experiments in VEST

Pre-ionization with Low-Field-Side Launch EC/EBW Heating

• LFS perpendicular injection of ECW 10 kW – Over-dense plasma (X-wave tunneling) – Parametric decay with mode conversion – High density with collisional heating near UHR

J.G. Jo et al., Phys. Plasmas 24 012103 (2017)

• TPC configuration for Pre-ionization – Higher density plasma with broad density profile – Different density profile with Bt strength

Low TF (Bo ~ 0.5 kG) High TF (Bo ~ 1 kG)

13/36

DC Helicity Injection

DC Helicity Injection Start-up

J.Y. Park

Lower Gun Location • R = 0.25 m

• Z = - 0.804 m

Magnetic Field Structure • Stacking ratio, G = 7 ~ 8

• Multiplication, M = ~ 16

The Electron Gun for DC Helicity Injection • High current electron beam based on arc discharge w/ washer stacks

• Low impurity

A toroidal current of up to ~20 kA has been generated by the electron gun • Increased multiplication factor confirmed as current streams reconnect

• Very dynamic states of current sheet observed

401 ms 402 ms 403 ms 400.4 ms

The electron gun

400 402 404 406 408 410 412 414 416-10

-9

-8

-7

-60

2

4

6

80.0

0.5

1.0

1.5

2.00.0

-0.2-0.4-0.6-0.8-1.0

0

5

10

15

20

Vacuum field @ Z = 0, R= 0.089

Bz [

mT

]

Time [ms]

Plasma field @ Z = 0, R= 0.089

Bz [

mT

]

I inj [

kA

]

Vin

j [kV

]

I toro

idal [

kA

]

14/36

Preparation for Advanced Tokamak Studies

Studies for Advanced Tokamak

15/36


Scopes of Advanced Tokamak Studies in VEST

Fusion reactor requires

high beta (or Q) and high bootstrap current

Simultaneously

Alpha heating dominant (high Q)

Centrally peaked pressure profile

Confinement and Stability?

High power

neutral beam heating

High Bootstrap current fraction (high fb)

Hollow current density profile (low li)

Current density profile control

Bootstrap/EBW/LHFW Profile diagnostics

16/36


Experimental Research Direction for VEST

Advanced Tokamak Study Scenario

Powerful Core Neutral Beam Injection (NBI) resembling alpha heating

tackles simultaneous achievement of high beta and high bootstrap current

High beta limit in spherical torus favors high beta operation

+

High bootstrap current fraction reversed shear configuration

Spherical Torus with Reversed Shear (RS) may provide AT regime !

• Sufficient confinement from ITB formation by RS

• Possible high beta even with low li in RS due to low aspect ratio

Advanced Tokamak Regime in VEST*

Low toroidal field(0.1T) with high βN(~7)< βN,Menard(8.7) and Ip(0.08 MA).

Fully non-inductive CD with 80% bootstrap fraction may be possible with ~500kW.

High H factor(~1.2) needs to be attempted by forming ITB with MHD stability.

* Estimated by 0-D system code

High power NBI to center, forming RS in VEST!

17/36


Simulations for the VEST Advanced Tokamak Scenario

Total power loss is calculated using NUBEAM simulations in various conditions (𝐈𝐏 = 𝟏𝟎𝟎 𝐤𝐀)

• Based on the result, R = 35 cm equilibrium is chosen for better NBI power absorption.

• 15 degree of the injection angle is chosen considering both power loss and engineering issue in VEST and its heating power profiles are calculated.

S.M. Yang, C.Y. Lee and Yong-Su Na

𝑬𝒃𝒆𝒂𝒎 R = 35 cm R = 40 cm

10𝑘𝑒𝑉 27% 33%

20𝑘𝑒𝑉 43% 70%

Total loss ( ne0 = 2 ∗ 1019 𝑚−3)

𝑃𝑒 , 𝑃𝑖 at 15° injection

𝑟/𝑎

Hea

tin

g p

ow

er

(

(ne0 = 2 ∗ 1019 𝑚−3)

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 Pe-20keV,600kW

Pe-10keV,150kW

Pi-20keV,600kW

Pi-10keV,150kW

18/36


Simulations for the VEST Advanced Tokamak Scenario

0-D system code analysis

Low toroidal field 0.1 T High beta N 7 Plasma current 80 kA

Non-inductive 100% Bootstrap fraction 80%

Required NB power 500 kW

1-D transport simulation NBI heating with NUBEAM code

(Density and equilibriums are given)

1D integrated modelling with ASTRA

code with TGLF module

(Density profile is given)

L-mode (edge at r=0.9) and H-mode

cases are compared

S.M. Yang, C.Y. Lee and Yong-Su Na

19/36


Preparation for Long Pulse Ohmic Discharges

S.C. Hong and J.Y. Park

TF with Ultra Capacitor from Battery PF in H-bridge circuit with Extra Capacitors

20/36


Development of Long Pulse Ohmic Discharges J.H. Yang and S.C. Kim

+10ms +20ms +30ms

-10ms 0ms

CE = 0.4

21/36


Discharge Improvement with Boronization

• Boronization during Helium Glow discharge cleaning – Carborane (C2B10H12) is used for non-toxic boronization – Water concentration recovery slowed down (RGA)

• Oxygen atom line emission (777.0 nm) reduced by 3 fold

– High density expected: More prefill gas (Hα line emission)

Diaphragm valve

Top flange (CF 6 inch)

Caborane oven

(baked 50°C)

Helium flow (MFC 2.0

sccm)

With great help from Dr. S.H. Hong (NFRI)

22/36

Neutral Beam Injection System

with ~500kW at 15keV

from KAERI

Thomson Scattering

with NdYAG laser

from SNU/NFRI/JNU/SogangU


Diagnostics, Heating and Current Drive Systems in VEST

Low Hybrid Fast Wave H/CD

with ~20kW at 500MHz

from KAERI/KAPRA/KWU

ECH

Electron Cyclotron/Bernstein

Wave H/CD

with ~30kW at 2.45GHz

from SNU/KAPRA

Interferometry

with 94GHz, multi-channel

from SNU/UNIST

Magnetics and Spectroscopy

Interferometer

Heating and Current Drive Systems Profile Diagnostic Systems

23/36

High power central heating

High Power NBI System : Dual Beam to Single Beam

NBI beam loss calculation with NUBEAM code

Too large orbit loss for counter-injection

Dual beam to single beam

Large shine through loss even for co-injection

Require high plasma density

5.0x1018

1.0x1019

1.5x1019

2.0x1019

2.5x1019

3.0x1019

0

10

20

30

40

50

60

70

80

90

100

Lo

ss R

ati

o [

%]

Target Plasma Density(#/m3)

counter-injection

co-injection

Beam at 20keV

S.H. Chung (KAERI)

24/36


High Power NBI System : System Design

3.1 m

Max Power

: 32.72 MW/m2

2.6 m

Max Power

: 35.16 MW/m2

1.8 m

Max Power

: 36.16 MW/m2

1.4 m

Max Power

: 32.77 MW/m2

0.7m

Max Power

: 28.12 MW/m2

Defl. angle, # of slits - 2 degree 29

- 1 degree 3

0 degree 13

1 degree 3

2 degree 29

Focusing beam profile calculation Beam power density at beamline

Calculated by BTR code

Slit beam focusing 3D beam line design

0

5

10

15

20

25

30

35

40

0 1 2 3

Maxim

um

Beam

Pow

er

Densi

ty [

MW

/m2]

Distance from Beam Exit [ m ]

2016-10-22

2016-09-23

2016-08-24

2016-07-25

2016-06-26

2016-05-27

2016-04-28

2016-03-29

25/36


High Power NBI System : Ion Source Test Completed

• Neutral beam injector – Co-directional – Design power: 500 kW

• Ion source test completed

– 750 kW at 15 keV • Under commissioning

– Power supply – Beamline

• Installation schedule

– March~April 2017

26/36

For high density plasma in fusion reactor

• Slow wave branch of LHW

→ Absorbed at the edge region

• Fast wave branch of LHW

→ Possible absorption at the core region

Proof of principle for current drive scheme

by fast wave branch of LHW in VEST


Core Heating and Current Drive with LHFW

S.H. Kim (KAERI)

Possible propagation regime in CMA diagram

- FW launching density ~ f(n||, w, wce)

- FW-SW confluence density ~ f(n||, wce)

FW path

Accessibility for Lower Hybrid Fast Wave(LHFW)

27/36

In collaboration with KAERI and Kwangwoon Univ.

• LH Fast Wave – Good core

accessibility – Propagation at

low density

• Hardware – UHF Klystron – Combline antenna

N|| = 3-5 / 500 MHz


LHFW H/CD Simulation and Component Preparation

28/36


EBW Heating and Current Drive with XB Mode Conversion

GENRAY, CQL3D, mode conversion codes are used

Perpendicular, LFS, X-mode injection

• Short distance between R(X) cut-off and UHR

• High XB mode conversion efficiency with low n||

• Good central heating and current drive expected

► Core heating and current drive with XB mode conversion

Absorption near EC fundamental resonance

EBW propagation

ECR

S.H. Kim (KAERI)

29/40 Status and plan for VEST

EBW heating (6kW cw+10kW pulse at 402ms) on Ohmic plasmas


EBW Heating with XB Mode Conversion

H.Y. Lee

400 401 402 403 404 405 406 407 408 409 410 411 4120

5

10

15

20

25

30

35

40

45

50

55

60

Pla

sm

a C

urr

en

t (k

A)

Time (ms)

off

on

• Boronization (impurity removal) and higher

plasma current (higher electron temperature)

• No change along the electron density profile

with additional MW – no collisional damping

• Plasma pulse duration is increased

• Electron temperature rises two times near 3rd

harmonics ECR resonance : the first

resonance position after mode conversion

30/36


VEST Diagnostic Systems

Diagnostic Method Purpose Remarks

Magnetic Diagnostics

Rogowski Coil Plasma current & eddy

current 3 out-vessel & in-vessel coils

Pick-up Coil & Flux Loop

Bz, Br & Loop voltage, flux

56 pick-up coils 9 loops

Magnetic Probe Array

Bz, Br measurement inside plasma

Movable single array

Probes Electrostatic Probe Radial profile of Te, ne

2 Triple Probes Mach probe

Optical Diagnostics

Fast CCD camera Visible Image 20kHz

Hα monitoring Hα Hα filter+ Photodiode

Impurity monitoring O & C lines Spectrometer

Interferometry Line averaged ne 94GHz, multi-channel

Reflectometry Radial profile of ne Edge density profile

EBE radiometer Core, edge Te BX mode conversion

Charge Exchange Spectroscopy

Rotation and Ti DNB

Thomson Scattering Te, ne profile NdYAG laser

31/36

Profile diagnostics

Movable Magnetic Probe Array System

J.H. Yang

• Equilibrium reconstruction with internal probe data • Direct derivation of JT(R) – w/o vertical displacement – Plasma degradation ~ 10%

J.H. Yang et al., Rev. Sci. Instrum. 83 10D721 (2012)

J.H. Yang et al., Rev. Sci. Instrum. 85 11D809 (2014)

32/36

Profile diagnostics

94 GHz Multi-channel Interferometer

J. Wang

• Microwave system with heterodyne interferometer

• 3.2 GHz IF (avoid EC noise)

– EC 2.45 GHz

• Single chord multi-chord

In collaboration with UNIST and KNU

33/36

Profile diagnostics

Thomson Scattering System

D.Y. Kim and Y.G. Kim Y.-G. Kim et al., Fus. Eng. Des. 96-97 882-885 (2015)

• Measurement target – ne: 5×1018 m-3

– Te: 10-200 eV • Laser energy: 0.65 J/pulse

In collaboration with NFRI and Seogang Univ.

34/36 Y.S. Kim, S.G. Ham and K.H. Lee

• Ion temperature and rotation measurement

– Dichronic beam splitter: Hα and HeII share collecting optics

– Transmission grating

• He2+ density measurement

• w/wo DNB

• Fabry-Perot interferometry

Profile diagnostics

Charge Exchange Spectroscopy/Beam Emission Spectroscopy

35/36

Future Research Plans

Long-term Research Plans

Reactor-relevant Advance Tokamak research

AT scenario for high beta and steady-state operation

Innovative divertor to handle long-pulse high-performance operation

Energetic particle transport …

36/36

Summary

VEST has achieved successful ohmic operation with plasma currents of up to ~100 kA, elongation

of ~ 1.6 and safety factor of ~6 with efficient ECH pre-ionization.

Efficient start-up method is developed with TPC(Trapped particle configuration) and EBW heating.

EBW heating with direct XB mode conversion from LFS launching by generating over-dense plasma in

the pre-ionization phase.

Enhanced pre-ionization with TPC improves start-up with low loop voltage, low ECH power, and wider

pressure window.

DC helicity injection startup experiments generate plasma current of ~20 kA.

Preparation for the study of Advanced Tokamak is progressing

VEST Advanced Tokamak scenario is simulated with transport codes.

Long-pulse ohmic discharges are being prepared by improving PF coil power system.

High power (~ 500 kW) NBI system is under development.

EBW/LHFW heating and current drive experiments are under preparation by performing simulations and

constructing hardware systems.

Profile diagnostics are under preparation

Reactor-relevant AT research by developing both AT scenario and innovative divertor concept will

be continued.

37/36

19th International Spherical Torus Workshop (ISTW 2017) will be held at

Seoul National University, 19-22 September 2017

Previous ISTWs were held in Oak Ridge (1994), Princeton (1995), Culham (1996), St. Petersburg (1997), Tokyo (1998),

Seattle (1999), Sao Jose dos Campos (2001), Princeton (2002), Culham (2003), Kyoto (2004), St. Petersburg (2005),

Chengdu (2006), Fukuoka (2007), Frascati (2008), Madison (2009), Toki (2011), York (2013), and Princeton (2015).

http://istw-2015.pppl.gov/config/pagetemplates/home-st-workshgop/World_STs.png?attredirects=0

38/36

Thank you for your attention !

39/36

VEST Discharge Status : 100kA with Boronization (#14945) Discharge Status (Shot #14945)

• Kinetic plasma parameters from magnetics

– Spitzer temperature*: ~200 eV

– Diamagnetic flux Stored energy (perpendicular) <1 kJ

– Estimated plasma density: ~2×1019 m-3

• Estimation from triple probe measurements

– ~ 1.5×1019 m-3 at 100 kA from ~ 8×1018 m-3 at 60 kA

*Zσ = 3 / trapped particle fraction 88%

Greenwald density limit: ~3×1019 m-3

40/36


Current initiation and field structure

-3

-2

-1

0

0

2

4

6

8

10

390 392 394 396 398 400 402 404 406 408 410

-1

0

1

2

PF1 Current

PF3&4 Current

PF9 Current

PF

Cu

rre

nt

[kA

]

w/ Trapped Particle Configuration (PF3&4)

w/o Trapped Particle Configuration (PF3&4)

Pla

sm

a C

urr

en

t [k

A]



Flu

x L

oo

p [

V]

Time [ms]

The field structure with no Ip does not provide stable field structure.

#5252 / #5255 Ip~8kA No Ip

41/36


Current initiation and field structure

-3

-2

-1

0

0

2

4

6

8

10

390 392 394 396 398 400 402 404 406 408 410

-1

0

1

2

PF1 Current

PF3&4 Current

PF9 Current

PF

Cu

rre

nt

[kA

]



Pla

sm

a C

urr

en

t [k

A]



Flu

x L

oo

p [

V]

Time [ms] #5252 / #5255 Ip~8kA No Ip

It was observed that negligible plasma current even with better Et*Bt/Bp and sufficient Vloop.

42/36


Decay Index for each scenario

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Decay In

dex

Major Radius [m]

Trapped Particle

Conventional w/ stable Bv

Conventional

Trapped Particle Configuration Stable Bv only Field Null

• Conventional field null configuration

does not provide stable Bv structure

at the onset of loop voltage

• The other scenarios provide stable Bv

structure at the onset of loop voltage

0 3 / 2

z

z

n

BRn

B R

43/36


Efficient volt-second consumption

0

5

10

15

20

25

400.0 400.2 400.4 400.6 400.8 401.0 401.2 401.40

2

4

6

8

Pla

sm

a C

urr

en

t [k

A] #8556

Vo

lt-s

ec

on

d [

mV

s]

Time [ms]

0

5

10

15

20

25

400.0 400.5 401.0 401.5 402.00

2

4

6

8

Pla

sm

a C

urr

en

t [k

A] #8553

Vo

lt-s

ec

on

d [

mV

s]

Time [ms]

4 mV∙s

6.9 mV∙s

1. When sufficient pre-ionization and proper field structure is provided wasted volt-second is

not shown in field null configuration.

2. Use of trapped particle configuration further reduces the required volt-second

1. Enhanced pre-ionization by TPC

2. Stable Bv at the onset of Vloop

1. Stable Bv at the onset of Vloop

Trapped Particle Configuration Stable Bv

0

5

10

15

20

25

401.5 402.0 402.5 403.0 403.5 404.0 404.5 405.00

2

4

6

8

Pla

sm

a C

urr

en

t [k

A] #8230

Vo

lt-s

eco

nd

[m

Vs]

Time [ms]

8.5 mV∙s

Wasted V∙s

1. Degraded pre-ionization

2. Not stable Bv at the onset of Vloop

Conventional

44/36



Efficient and robust tokamak start-up demonstrated

Y. An et al., Nucl. Fusion 57 016001 (2017)

Trapped particle Field null

• Mirror like magnetic field configuration – Enhanced particle confinement – Inherently stable decay index structure – Wider operation range of ECH power, pressure and low Vloop

45/36

Ohmic Start-up with ECH Pre-ionization

Higher dIp/dt and Wider Operation Regimes with TPC

1 2 3 4 5 6

0

1

2

3

4

5

6

7

dI p

/dt

[kA

/ms

]

ECH Power [kW]

with TPC

without TPC

No current initiation

10-5

10-4

-1

0

1

2

3

4

5

6

7

8

dI p

/dt

[kA

/ms]

Filling Pressure [Torr]

With TPC

Without TPC

No current initiation

20 40 60 80 100 120 140 160 180

1

2

3

4

5

6

7

8

9

10 with TPC

without TPC

dI p

/dt

[kA

/ms]

Et*B

t/B

p [V/m] at R=0.4m

Higher dIp/dt under TPC with identical Vloop and EtBt/Bp

Wider operating windows for gas pressure and ECH power

Y.H. An

46/36


Solenoid-free Start-up from Outboard with Outer Poloidal Field Coils H.Y. Lee

The closed flux surfaces have not been formed in the cases of

both 6kW and 16 kW due to their high resistivity of > 1.0 E-5.

In the case of the resistivity of 1.0 E-5, the closed flux surface has

small minor radius 3~4 cm with very low Ip.

Once current column is formed, inward moving current can grow

by utilizing decreasing external inductance even with small E field.

Plasma current may be initiated with resistivity of less than 1.0 E-5.

To reach target resistivity, higher ECH power of ~30kW

may be needed to have sufficient density and

temperature in the pre-ionization phase.

Resistivity

1E-5

Resistivity

5E-6

47/36


Widely used Field Null Configuration (Breakdown/avalanche)

The field null configuration has been generally used even for the ECH-assisted start-up of tokamaks.

KSTAR

[Y.S. Bae et al 2009 Nucl. Fusion 49 022001]

Frascati Tokamak Upgrade

[G. Granucci et al, Nucl. Fusion 51 (2011) 073042]

Field null formation to maximize connection length and consequently sufficient EtBt/Bp

Null quality : size, sustaining time and location loop voltage and pre-ionization

Notes:

KSTAR: not much help with higher ECH power in the ECH-assisted start-up

DIII-D: vertical field pre-bias helps ohmic start-up with ECH pre-ionization

DIII-D

[NUCLEAR FUSION, Vol. 31. No. 11 (1991)]

48/36


Mirror Ratio and Decay Index for Efficient Start-up in TPC

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

Decay In

dex

Major Radius [m]

Trapped Particle

Conventional w/ stable Bv

Conventional

Stable Bv only Field Null

• Conventional field null configuration

does not provide stable Bv structure

at the onset of loop voltage

• Other scenarios provide stable Bv

structure at the onset of loop voltage

0 3 / 2

z

z

n

BRn

B R

• Efficient start-up with better pre-

ionization at high mirror ratio


49/36

Ohmic Start-up with ECH Pre-ionization

Double Swing Start-ups with TPC and FNC

J.W. Lee

PF#1 Double swing TPC

402 404 406 408 410 412 414

0

20

40

60

80

100

Pla

sm

a c

urr

en

t [k

A]

Time [ms]

shot #14458, Full flux TPC

shot #14597, Full flux FNC, same day

PF#1 full flux startup using field null vs. TPC

Null: PF#1+PF#10, TPC: PF#1+PF#5

Fast plasma current ramp-up

Efficient ECH-assisted start-up using trapped particle configuration in VEST & KSTAR

and application for ITER

J. Lee, J. Kim, Y.H. An, M.-G. Yoo, Y.S. Hwang and Y.-S. Na (SNU and NFRI in Korea)

Reliable ECH-assisted start-up with

ITER-relevant Et in KSTAR

• BRIS-less start-up to counter Ip

operation for various physics

studies in KSTAR

• ITER-relevant Et level of less

than 0.3 V/m

• Unstable start-up with field null

even with ECH-assistance

• Reliable low Et start-up with pre-

ionization plasmas under TPC

Preliminary ITER scenario development using TPC

• Scenario development based on ITER_c_33NHXN_v3_4, appendix 6, half charging CS and P/S configuration B

• With consideration of eddy current evolution driven by PF coils on conducting structure

• Almost identical operation of solenoid coils, and newly determining the outer PF currents for TPC configuration

• About 0.2 s after solenoid swing, achieving the Et for Ip formation based on KSTAR result

• Save V∙s and avoid operating solenoid coils up to its engineering limits, Et of 0.3 V/m

Bp and pol. flux at 0.2 s

51/36


Simulation Results for LHFW Heating and Current Drive S.H. Kim (KAERI)

TORIC Full Wave simulation : Absorption

GENRAY-CQL3D simulation : Current Drive

Ez field on Poloidal C.X. Power absorption dist. Driven current profile

LHFW ray tracing LHSW ray tracing Velocity dist. on phase space by CQL3D

Driven current profile

1D full wave coupling code

LHFW RF system and

operation parameters

Coupling cal. condition Coupling efficiency vs. den.

- Frequency = ~ 500 MHz (UHF)

- N∥ = 3 ~ 5, B0 = 0.2 T

- ne0 ~ 1x1018#/m3, Te0 ~ 1 keV

☆ In addition, 1D full wave code is developed to calculate

condition for efficient LHFW coupling: nb~3x1017#/m3

52/36


LHFW Components

Klystron

The klystron is prepared by refurbishing an

old UHF broadcasting system of Korea which

has been hold by SNU and KAPRA.

Combline Antenna

S.H. Kim (KAERI) and B.J. Lee (KwangWoon Univ.)

Frequency 470~520 MHz

VSWR < 3:1

Peak n|| 3.25

Antenna size length = 486 mm, thickness = 36mm

Ey/Ez ratio 4~30 (average of 71.35%)

Peak E-field < 24.4kV/cm

53/22

LHY_JMVQ_DEC17

Joint meeting on VEST & QUEST on December 17th 2015, at Kyushu Univ.

-6 -4 -2 0 2 4 6

700

750

800

850

900

950

Ma

gn

etic

Fie

ld (

G)

Distance (cm)

Radial FieldQuartz Quartz

HFS LFS 875G

`

W

R

2

8

4

Pump

System

3 Stub

Tuner

2.45 GHz

Magnetron

Quartz Tube

Stainless Steel Wall

MW Loss

Protection

Bent

Magnetic

Field(BT)

LFS

Injection

HFS Injection

EBW Heating Experiments in linear device

Experimental Setup

Objectives : To find the feasibility of ECH from EBW

via XB mode conversion.

Small cylindrical device with axial magnetic field bent

similar to tokamak

Possible to change the direction of MW injection mode

– LFS&HFS

Easily changeable of magnetic field and MW power –

movable location of ECR

Antenna : open waveguide(WR284)

54/22

LHY_JMVQ_DEC17

Joint meeting on VEST & QUEST on December 17th 2015, at Kyushu Univ.

Diagnostics – measured at r = 0

The plasma with higher electron density generates from

LFS X mode than from HFS X mode.

As shown in CMA diagram, LFSX/HFSX injection does not

exceed R/L cutoff but in the experiment LFSX injection

overcomes the R cutoff via mode conversion.

Feasibility of EBW via direct XB mode conversion

300 400 500 600 700 800 9000.0

1.5

3.0

4.5

6.0

7.5

ne

[1

01

7m

-3]

Microwave Power [W]

High Field Side X-mode

Low Field Side X-mode

L-cutoff density

1.45 x 1017

m-3

ECH Experiment in linear device

Over-dense plasma generation

55/36

30 35 40 45 50 55 60 65 70 75 80 853

6

9

12

15

18

21

24

Te [

eV

]

R [cm]

X-mode_2kW

X-mode_3kW

X-mode_4kW

X-mode_6kW

ECR

30 35 40 45 50 55 60 65 70 75 80 850.0

0.2

0.4

0.6

0.8

1.0

1.2

nUH

nR

ne [10

17m

-3]

R [cm]

X-mode_2kW

X-mode_3kW

X-mode_4kW

X-mode_6kW

ECR


Analysis on VEST pre-ionization plasma

UHR

Power absorption near the UHR(ne) by collisional damping and ECR(Te) by collisionless damping respectively.

Shorten density scale length by steep density gradient near outer limiter

1D full wave code has been developed and XB mode conversion efficiency is estimated to be ~20%.

ECR

J.G. Jo and S.H.Kim

56/36


1D full wave simulation for QUEST

J.G. Jo and S.H.Kim

1D Full wave simulation

XB Mode conversion efficiency calculation

Condition : MW frequency, toroidal magnetic field, density profile

Kim, S. H., et al. Physics of Plasmas 21(6) (2014).

"One-dimensional full wave simulation on XB

mode conversion in electron cyclotron heating."

Example : 1D Full wave simulation

QUEST Simulation (8.2 GHz)

Density profile: Assumption

- Peak density (2e18, 5.5e18 #/m3)

XB Mode conversion coefficient

- 2e18 #/m3: ~1%

- 5.5e18 #/m3: ~38% 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

0.00E+000

5.00E+017

1.00E+018

1.50E+018

2.00E+018

2.50E+018

3.00E+018

3.50E+018

4.00E+018

4.50E+018

5.00E+018

5.50E+018

6.00E+018

8.2 Ghz

2nd

8.2 Ghz

1st

De

nsity

Distance (m)

R cutoff

UHR

L cutoff

O Cutoff

2e18

5e18

28 Ghz

2nd

57/36 Y.S. Kim and S.G. Ham

Profile diagnostics

Imaging Fabry-Perot interferometry

• Gas and Ion temperature measurement

– Hα : Gas temperature

– Ov : Ion temperature at core

• High resolution without scanning

• Preliminary experiment is in progress

• Measure Isotope shift (Hα Dα lamp)

– Take pictures by Nikon camera

(f = 120 mm, mirror gap = 1.5 mm)

– Real value : 0.178 nm

– Measured value : 0.172 ± 0.014 nm

58/36

Future Research Plans

Innovative Divertor Studies

K.S. Chung and K.J. Chung

Direct Energy Conversion concept applied in VEST divertor

Conceptual design of DEC applied on VEST

overview of versatile experiment spherical torus …...trapped particle configuration (tpc)...

Documents